This invention relates to an apparatus for controlling drag on land transport vehicles, prticularly tractor trailer trucks. Much progress has been made in reducing drag on tractor trailer trucks in the recent past. Substantial increases in diesel fuel prices have made such improvements necessary. New truck designs are much more streamlined than before, and deflectors have been designed to fit older trucks. One of the most significant reductions in drag was achieved when air deflectors were mounted on the cab roof of the tractors to deflect air over the top of the trailer. This deflection prevents air from passing unimpeded over the top of the relatively low roof of the tractor and impacting at right angles the front of the much taller trailer which projects over the top of the tractor cab roof.
Skirts have also been added to the rear sides of tractors to deflect air past the front of and along the sides of the trailer, instead of curling around the rear of the tractor and, as with the air over the top of the cab roof, impacting the front of the trailer. There are, however, drawbacks to these improvements, Most importantly, the reduced drag increases the braking distance of the vehicle, since drag is providing less stopping assistance. This is a particular problem in mountainous terrain where the air is less dense to begin with, and where trucks must brake constantly when traveling downhill.
In addition, while a roof mounted air deflector reduces drag when a tractor is pulling a conventional trailer which extends up over the height of the cab roof, it actually increases drag when the tractor is pulling no load, or a lower profile trailer such as a flatbed. Therefore, it is desirable to provide means of increasing or decreasing drag as is desirable to optimize the overall efficiency of the vehicle.